1. Field of the Invention
This invention relates generally to the control of longwall type mining machinery, and particularly to a system for automating the positioning of coal shearing drums whereby an optimum thickness of a coal seam is sheared and smooth controlled cuts obtained.
2. Description of Related Art
As is well known, longwall mining is one of the most productive underground mining methods in use today, employing the most technically advanced mining equipment. In longwall mining, the area to be mined is divided into blocks, typically 500 to 1,000 feet across and 5,000 feet long. Two access tunnels are drilled on either side of the block, and a face tunnel, along a seam of coal, is cut between access tunnels. Roof supports and a track for a longwall coal cutting machine, or longwall shearer, are installed through the face tunnel. The cutting machine makes a cut into the face each time it traverses the track from access tunnel to access tunnel. The cut coal is carried by a conveyor back to an access tunnel for removal. After each traverse of the cutting machine, the roof supports and track are advanced toward the new face of the seam for the next cut. Coal seams mined in this manner are usually in the range of 48 to 144 inches in vertical thickness, with an adjacent region above and below a seam of varied composition. Frequently, the adjacent region is of clay or other unstable material, and for this reason, the tunnel in which the cutting operation occurs is often formed in the coal with an adequate ceiling and floor thickness of coal to maintain the integrity of the tunnel above and below the cutting machine during its pass through the tunnel. As conditions may require, from two the six inches of coal may be left on the ceiling and/or the floor. For example, sufficient thickness of coal on the floor is necessary to support the weight of the shearing machine and roof supports in areas where soft floor materials are prevalent. Alternately, if a greater amount of coal is left than actually required, the economic impact is substantial and can run into the millions of dollars if such deviations persist during significant mining operations. In addition, automated positioning of the cutting drums can result in smoother cuts with less steps, etc., which reduces equipment breakage and maintenance. Also, operating personnel can move away from the cutters, reducing their exposure to coal dust and flying debris.
The obvious problem is how to accurately track and follow a coal seam and its boundaries. If coal seams were level, the task would be relatively simple but they are not. Instead, they largely tend to follow a constantly changing slope and, in fact, in many instances conform to sine waves with amplitudes typically running from 3 to 12 feet and periods of typically 50 to 100 feet. Another and related factor is that the cutting or shearing drums of the cutting machine extend significantly beyond the ends of the machine and cut to the side of the machine and thus form a tunnel for the next pass by the machine. The problems that arise will be appreciated from a brief examination of operation.
Assume first that an initial tunnel is precisely cut between access tunnels along a seam and that it has a known sine wave contour. A track, made of sections generally referred to as pans, is laid through the tunnel beside a face of the seam to be sheared. The shearing machine is then positioned on the track, and its initial task is to make a first cut in the face of the seam, beside the initial tunnel. Ideally, a cutting drum would be set at one elevation to cut either the floor or ceiling of a new cut which is identical with the ceiling or floor elevation of the initial tunnel. Unfortunately, while the machine body is typically supported by its four rollers in the initial tunnel, a cutting drum is in advance of the body of the machine by several feet as it cuts, in effect, a new tunnel. The result is that the elevation of the body is in terms of the slope of the tunnel under it, whereas the cut made by the cutter, to the side of the initial cut, is in advance of the body of the machine. As a result, a single relative position of the drum with respect to the machine will not produce side-by-side like contoured cuts. If one does proceed in a single position of the drum, there will be produced a new cut which will follow a sine wave, but this sine wave will be longitudinally displaced with respect to the sine wave contour of the original tunnel, and it thus will not conform to the coal seam. To make matters worse, actual measurement of the position of the drum with respect to the floor is not possible because of a too-hostile environment for instrumentation.
Because of these factors, it is believed that a practical system for automated control of a longwall shearer has not been devised. As a result, the contour of cutting is simply left up the eye of the operator of the machine, aided by occasional core drillings which spot examine the thickness of coal and guide the operator in adjusting a cut. Experience, however, has shown that these means do not allow an operator to guide the cutting operation under the circumstances described such that efficient and safe operation can be consistently obtained. Accordingly, there remains a very basic need for some form of effective automated aid to remove a significant element of the guesswork which the operator must now engage in in the correction of a longwall cutting machine.